The document U.S. Pat. No. 6,122,424 describes an optical cable which has a plurality of optical waveguides, a sleeve surrounding the optical waveguides, and a cable sheath surrounding the sleeve. A component which contains a flame-retardant substance and a water-absorbent substance is arranged between the sleeve and the cable sheath. Antimony-trioxide, chlorinated paraffins, bromium-phosphorous compounds, aluminium trihydrate, zinc borate, magnesium hydroxide and tin compounds are specified as suitable flame-retardant substances. For suitable water-absorbent substances, reference is made to the document U.S. Pat. No. 4,913,517, which specifies super-absorbent materials based on polyacrylic acids, polyacrylonitrile or polyvinyl alcohol.
An optical cable is known from the document U.S. Pat. No. 6,501,887 B1, which has a plurality of ribbons of optical waveguides, a sleeve surrounding the ribbon, and a cable sheath surrounding the sleeve. A layer composed of an inherently flame-retardant material is arranged between the sleeve and the cable sheath, and is covered by a layer composed of a material which can expand. Natural or synthetic micas, glasses, polyimides and aramides are specified as suitable flame-retardant materials. Super-absorbent polymers and in particular partially cross-linked acrylate polymers are specified as suitable materials which can expand.
A fire protection element which is suitable for installation in a building is known from the document DE 198 37 998 A1, which has a core composed of a water glass which is enclosed in a gastight and watertight manner with a residual water content of 20 to 40 per cent by weight and a sheath composed of a polyurethane foam with a foam weight of 10 to 100 grams per liter. The water glass has a mol ratio of SiO2/Na2O of 3.41, and 25 per cent by weight of water of crystallization. When the water glass is heated to a temperature of about 200° C., then the water of crystallization is emitted in the form of water vapour resulting in a solid mineral foam, which withstands temperatures of 1000° C. for up to 240 minutes.
An optical cable which has a plurality of optical waveguides and a cable sheath surrounding the optical waveguides is known from the document US 2003/0031818 A1. The cable sheath contains a halogen-free plastic. Polyethylene, polypropylene and polyurethane are specified as suitable halogen-free plastics. In order to improve the fire protection characteristics, the cable sheath also contains a foam-forming substance and a glass-forming substance. Water glasses, expandable micas, compressed graphites and mixtures of these substances are specified as suitable foam-forming substances. Salts of boric acid, salts of silicide acid and salts of phosphoric acid are specified as suitable glass-forming substances.
Further known flame-retardant substances which are introduced into a cable sheath are, for example, aluminium hydroxide or magnesium hydroxide. The introduction of flame-retardant substances into the cable sheath may have a disadvantageous influence on other characteristics of the cable sheath. For example, the resistance to tearing of the cable sheath may be reduced. This effect may in fact be desirable. However, in general, the cable sheath should have good resistance to tearing, in order to effectively protect the cable interior.